375 mg: Increased plasma conc w/ inhibitors of CYP3A4; CYP2D6 inhibitors. Increased AUC w/ ketoconazole. Contraindicated w/ potent CYP3A4 inhibitors (eg, itraconazole, ketoconazole, voriconazole, parconazole, HIV PIs, clarithromycin, telithromycin, nefazodone); grapefruit juice. Dose-dependent increases in ave steady-state conc w/ diltiazem & other moderately potent CYP3A4 inhibitors (eg, erythromycin, fluconazole). Increased plasma levels w/ inhibitors of P-gp (eg, ciclosporin, verapamil). Decreased steady-state conc w/ rifampicin. Avoid initiation of treatment during administration of inducers of CYP3A4 (eg, rifampicin, phenytoin, phenobarb, carbamazepine, St. John's wort). Increased steady-state plasma conc w/ potent CYP2D6 inhibitor paroxetine. May increase plasma conc of P-gp or CYP3A4 substrates; sensitive CYP3A4 substrates (eg, simvastatin, lovastatin) & CYP3A4 substrates w/ narrow therapeutic range (eg, ciclosporin, tacrolimus, sirolimus, everolimus). Tissue distribution of drugs which are transported by P-gp may be increased. Increased plasma conc of metoprolol. Exposure to metoprolol or other CYP2D6 substrates (eg, propafenone & flecainide or, TCAs & antipsychotics) may be increased during co-administration. Caution is advised during co-administration w/ CYP2B6 substrates (eg, bupropion, efavirenz, cyclophosphamide). Increased plasma digoxin conc. Increased plasma conc of simvastatin lactone, simvastatin acid; tacrolimus. Increased Cmax & AUC of atorvastatin. Dose limitation of other statins, metabolized by CYP3A4 (eg, lovastatin). Increased plasma exposure of metformin & other OCT2 substrates, including pindolol & varenicline. Concomitant treatment w/ other drugs known to prolong QTc interval including certain antihistamines (eg, terfenadine, astemizole, mizolastine), antiarrhythmics (eg, quinidine, disopyramide, procainamide), erythromycin, & TCAs (eg, imipramine, doxepin, amitriptyline) may give rise to pharmacodynamic interaction & increase the possible risk of ventricular arrhythmias. 500 mg: Do not use w/ strong CYP3A inhibitors including ketoconazole, itraconazole, clarithromycin, nefazodone, nelfinavir, ritonavir, indinavir & saquinavir. Increased ave steady-state plasma conc of 3.2-fold w/ ketoconazole (200 mg bid). Limit dose to 500 mg bid in patients on moderate CYP3A inhibitors including diltiazem, verapamil, aprepitant, erythromycin, fluconazole, & grapefruit juice or grapefruit-containing products. Increased steady-state plasma conc of about 2-fold w/ diltiazem (180-360 mg daily) & verapamil (120 mg tid). Down-titrate dose in patients concomitantly treated w/ P-gp inhibitors eg, cyclosporine. Avoid co-administration w/ CYP3A inducers eg, rifampin, rifabutin, rifapentin, phenobarb, phenytoin, carbamazepine & St. John's wort. Decreased plasma conc w/ rifampin (600 mg once daily). Increased conc of 1.2-fold w/ paroxetine. Increased plasma levels about 2-fold of simvastatin. Elevated plasma conc about 1.5-fold of digoxin. May require lower doses of CYP2D6 substrates.